testsuite: Remove pgrep ?15 char test

The referenced commit removed the warning for using pgrep with over
15 characters. The check for this warning needs to also be removed.

References:
 commit c32ab58b94

testsuite/pgrep.test/pgrep.exp

@@ -118,9 +118,5 @@ set test "pgrep does not match substring with exact"
 spawn $pgrep -x $testproc_trim
 expect_blank $test
 
-set test "pgrep with long non-matching pattern gives warning"
-spawn $pgrep gnome-session-bi
-expect_pass "$test" "pattern that searches for process name longer than 15 characters will result in zero matches"
-
 # Cleanup
 kill_testproc