QA: Remove unused update postponements

Clang has been updated, and the Rust dependencies aren't checked by
updatecheck.py any more.

qa/zcash/postponed-updates.txt

@@ -4,22 +4,6 @@
 # bdb 18.1.40 2020-09-01
 #
 
-# Clang is pinned to a version that matches the Rust version.
-# This would be Clang 9, but for 4.1.0 we are using Clang 8 and postponing the upgrade.
-# libc++ is pinned to the same version as Clang.
-native_clang 8.0.1 2020-12-01
-native_clang 9.0.0 2020-12-01
-native_clang 9.0.1 2020-12-01
-native_clang 10.0.0 2021-01-20
-native_clang 10.0.1 2021-01-20
-native_clang 11.0.0 2021-01-20
-libcxx 8.0.1 2020-12-01
-libcxx 9.0.0 2020-12-01
-libcxx 9.0.1 2020-12-01
-libcxx 10.0.0 2021-01-20
-libcxx 10.0.1 2021-01-20
-libcxx 11.0.0 2021-01-20
-
 native_ccache 4.0 2021-01-20
 
 bdb 18.1.40 2021-01-20
@@ -33,64 +17,3 @@ native_b2 1.75.0 2021-03-01
 
 # Google Test 1.10.0 requires adding CMake to the depends system.
 googletest 1.10.0 2021-01-20
-
-# The aes crate depends on "aesni ^0.8"
-crate_aesni 0.9.0 2020-11-01
-
-# The bigint crate depends on "crunchy ^0.1.5"
-crate_crunchy 0.2.1 2020-11-01
-crate_crunchy 0.2.2 2020-11-01
-
-# These are being deferred until after 4.0.0 to avoid blocking the release.
-crate_time 0.1.44 2021-01-20
-crate_redox_users 0.3.5 2021-01-20
-
-# The chrono crate depends on "time ^0.1.43", and is highly unlikely to
-# upgrade to v0.2: https://github.com/chronotope/chrono/issues/400
-crate_time 0.2.0 2021-02-01
-crate_time 0.2.1 2021-02-01
-crate_time 0.2.2 2021-02-01
-crate_time 0.2.3 2021-02-01
-crate_time 0.2.4 2021-02-01
-crate_time 0.2.5 2021-02-01
-crate_time 0.2.6 2021-02-01
-crate_time 0.2.7 2021-02-01
-crate_time 0.2.8 2021-02-01
-crate_time 0.2.9 2021-02-01
-crate_time 0.2.10 2021-02-01
-crate_time 0.2.11 2021-02-01
-crate_time 0.2.12 2021-02-01
-crate_time 0.2.13 2021-02-01
-crate_time 0.2.14 2021-02-01
-crate_time 0.2.15 2021-02-01
-crate_time 0.2.16 2021-02-01
-
-# The futures-cpupool crate depends on "futures ^0.1", which was last
-# updated 3 years ago. We plan to move away from it when we refactor
-# bellman's multicore support.
-crate_futures 0.2.0 2021-02-01
-crate_futures 0.2.1 2021-02-01
-crate_futures 0.3.0 2021-02-01
-crate_futures 0.3.1 2021-02-01
-crate_futures 0.3.2 2021-02-01
-crate_futures 0.3.3 2021-02-01
-crate_futures 0.3.4 2021-02-01
-crate_futures 0.3.5 2021-02-01
-
-# The redox_users crate depends on:
-# - "redox_syscall ^0.1"
-# - "rust-argon2 ^0.7", which in turn depends on "base64 ^0.11"
-#
-# redox_users 0.3.5 depends on rust-argon2 ^0.8 which will address some of this.
-crate_base64 0.12.0 2020-11-01
-crate_base64 0.12.1 2020-11-01
-crate_base64 0.12.2 2020-11-01
-crate_base64 0.12.3 2020-11-01
-crate_redox_syscall 0.2.0 2020-11-01
-crate_redox_syscall 0.2.1 2020-11-01
-crate_rust_argon2 0.8.0 2020-11-01
-crate_rust_argon2 0.8.1 2020-11-01
-crate_rust_argon2 0.8.2 2020-11-01
-
-# tracing 0.1.20 was yanked.
-crate_tracing 0.1.20 9999-12-31