revparse: Remove error-prone, redundant test

Originally introduced in: 776a6a8e

This test case has recently been fixed in bdab2238, but that fix will only last for a year. Next year the same problem will crop up and the test will need to be re-edited.

This is not ideal as
- This test case becomes an unnecessary burden for developers
- Downstream distros or even just users who want to build older versions of libgit2 are guaranteed to have this test fail

Furthermore, this test case is entirely unnecessary, as the functionality that was originally (see 776a6a8e) intended to be tested is well-covered by subsequent tests which specify a date instead of a "x ago" specification.

tests/refs/revparse.c

@@ -400,7 +400,6 @@ void test_refs_revparse__date(void)
 	 * a65fedf HEAD@{1335806603 -0900}: commit:
 	 * be3563a HEAD@{1335806563 -0700}: clone: from /Users/ben/src/libgit2/tests/resour
 	 */
-	test_object("HEAD@{10 years ago}", NULL);
 
 	test_object("HEAD@{1 second}", "a65fedf39aefe402d3bb6e24df4d4f5fe4547750");
 	test_object("HEAD@{1 second ago}", "a65fedf39aefe402d3bb6e24df4d4f5fe4547750");