rustbuild: Remove --enable-llvm-clean-rebuild

[alexcrichton]

This was intended for bots back in the day where we'd persist caches of LLVM
builds across runs, but nowadays we don't do that on any of the bots so this
option is no longer necessary

[alexcrichton]

r? @Mark-Simulacrum

[rust-highfive]

r? @brson

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

[petrochenkov]

@bors r+ rollup

[bors]

📌 Commit eb1c44b has been approved by petrochenkov

[bors]

⌛ Testing commit eb1c44b with merge f3ff38af86bbb873cc089e603ccff95c999671e0...

[bors]

💔 Test failed - status-travis

[kennytm]

@bors retry

#43402 in x86_64-apple-darwin check

[01:38:10] failures:
[01:38:10] 
[01:38:10] ---- [run-make] run-make/issue-26092 stdout ----
[01:38:10] 	
[01:38:10] error: make failed
[01:38:10] status: exit code: 2
[01:38:10] command: "make"
[01:38:10] stdout:
[01:38:10] ------------------------------------------
[01:38:10] DYLD_LIBRARY_PATH="/Users/travis/build/rust-lang/rust/build/x86_64-apple-darwin/test/run-make/issue-26092.stage2-x86_64-apple-darwin:/Users/travis/build/rust-lang/rust/build/x86_64-apple-darwin/stage2/lib:" '/Users/travis/build/rust-lang/rust/build/x86_64-apple-darwin/stage2/bin/rustc' --out-dir /Users/travis/build/rust-lang/rust/build/x86_64-apple-darwin/test/run-make/issue-26092.stage2-x86_64-apple-darwin -L /Users/travis/build/rust-lang/rust/build/x86_64-apple-darwin/test/run-make/issue-26092.stage2-x86_64-apple-darwin  -o "" blank.rs 2>&1 | \
[01:38:10] 			grep -i 'No such file or directory'
[01:38:10] 
[01:38:10] ------------------------------------------
[01:38:10] stderr:
[01:38:10] ------------------------------------------
[01:38:10] make[1]: *** [all] Error 1
[01:38:10] 
[01:38:10] ------------------------------------------
[01:38:10] 
[01:38:10] thread '[run-make] run-make/issue-26092' panicked at 'explicit panic', src/tools/compiletest/src/runtest.rs:2499:8
[01:38:10] note: Run with `RUST_BACKTRACE=1` for a backtrace.
[01:38:10] 
[01:38:10] 
[01:38:10] failures:
[01:38:10]     [run-make] run-make/issue-26092
[01:38:10] 
[01:38:10] test result: �[31mFAILED�(B�[m. 156 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out
[01:38:10] 
[01:38:10] thread 'main' panicked at 'Some tests failed', src/tools/compiletest/src/main.rs:323:21

[bors]

⌛ Testing commit eb1c44b with merge beec479106a999be5f4b7834f3116b0819493a13...

[bors]

⌛ Testing commit eb1c44b with merge 2a6828e...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: petrochenkov
Pushing 2a6828e to master...