Remove unnecessary unsafe block around calls to discriminant_value

[tmiasko]

Since 63793 the discriminant_value intrinsic is safe to call. Remove
unnecessary unsafe block around calls to this intrinsic in built-in
derive macros.

[rust-highfive]

r? @varkor

(rust_highfive has picked a reviewer for you, use r? to override)

[jonas-schievink]

Seems like this might impact perf due to the reduced amount of code generated.

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit 50da126 with merge 90c516e222e0c4c6698cf58b124d30881de8bf3c...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 90c516e222e0c4c6698cf58b124d30881de8bf3c (90c516e222e0c4c6698cf58b124d30881de8bf3c)

[rust-timer]

Queued 90c516e222e0c4c6698cf58b124d30881de8bf3c with parent bc74dd7, future comparison URL.

[rust-timer]

Finished benchmarking try commit (90c516e222e0c4c6698cf58b124d30881de8bf3c): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never

[jonas-schievink]

@bors r+

[bors]

📌 Commit 50da126 has been approved by jonas-schievink

[bors]

⌛ Testing commit 50da126 with merge d6b5ffb...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: jonas-schievink
Pushing d6b5ffb to master...