[clang][test] Remove check that fails if SOURCE_DATE_EPOCH is set globally

The check for "no SOURCE_DATE_EPOCH" wasn't especially interesting, and
I am not aware of a _portable_ way to unset and environment variable in
a lit test. So remove it since it can fail if the build environment has
SOURCE_DATE_EPOCH set globally.

Differential Revision: https://reviews.llvm.org/D142511