[Arm] Remove duplicate CPU tests

There are some duplicate test lines in clang/test/Driver/arm-cortex-cpus.c.
Looks like these were duplicated from the corresponding v8.0a tests, which test
both "-target armv8" and "-target armv8a". "-target armv8.X" without the "a"
doesn't work for later versions though.

Several tests also specify the -mlittle-endian twice, which looks unintentional.

Remove duplicate test: -target arm -march=armv5 -mthumb

Differential Revision: https://reviews.llvm.org/D116415