Fix flaky symlink access time test.

Fix flaky symlink access time test.

last_write_time(sym, new_time) changes the modification time of the file
referenced by the symlink. But reading through the symlink may change the
symlinks's access time.

This meant the previous test that checked that the symlinks access
time was unchanged was incorrect and made the test flaky.

This patch removes this test (there really is no non-flaky way
to test that the new access time coorisponds to the time at which
the symlink was last dereferenced). This should unflake the test.

llvm-svn: 350478