gdb/testsuite: Revert commit 843f4d93576eef02139f7b1b3fa1cea7b0f286f1

gdb/testsuite: Revert commit 843f4d93576eef02139f7b1b3fa1cea7b0f286f1

Revert this commit:

  commit 843f4d93576eef02139f7b1b3fa1cea7b0f286f1
  Date:   Tue May 12 17:38:17 2020 +0100

      gdb/testsuite: Disable path and duplicate checks when parallel testing

Now that this commit has landed:

  commit c959562d9ba0b2eaf240c601b2c2fd49c42c1f2f
  Date:   Fri May 15 11:23:59 2020 +0100

      contrib: Update dg-extract-results.* from gcc

We can now make use of the mechanism for detecting paths in test names
and duplicate test names, even when we run tests in parallel.

gdb/testsuite/ChangeLog:

	* lib/check-test-names.exp: Remove code that prevents this file
	loading when tests are run in parallel.