Tests: Remove LibRegex benchmark test file that has become stale

This test file had #ifdef macros at the top that caused none of the
content to be compiled unless a developer manually wanted to run the
specific benchmarks within. As such, it has become stale. Remove it for
now, if someone wants to restore it in an always-runnable state, we can
restore the specific tests it's trying to benchmark.

Tests/LibRegex/CMakeLists.txt

@@ -1,5 +1,4 @@
 set(TEST_SOURCES
-    Benchmark.cpp
     Regex.cpp
     RegexLibC.cpp
 )