tests: remove obsolete uses of HGTEST_RUN_TESTS_PURE

342ab95a1f4b added arguments to run-tests.py calls to propagate --pure when
--with-hg is not used.

0622d6e134fb unified places to use --with-hg via rt(), which obviated the
need for passing HGTEST_RUN_TESTS_PURE.

tests/test-run-tests.t

@@ -689,7 +689,7 b' Mercurial source tree.'
   >   $ echo foo
   >   foo
   > EOF
-  $ rt $HGTEST_RUN_TESTS_PURE test-hghave.t
+  $ rt test-hghave.t
   .
   # Ran 1 tests, 0 skipped, 0 warned, 0 failed.
 
@@ -712,7 +712,7 b' running is placed.'
   >   #
   >   # check-code - a style and portability checker for Mercurial
   > EOF
-  $ rt $HGTEST_RUN_TESTS_PURE test-runtestdir.t
+  $ rt test-runtestdir.t
   .
   # Ran 1 tests, 0 skipped, 0 warned, 0 failed.
 
@@ -729,7 +729,7 b' test that TESTDIR is referred in PATH'
   >   $ custom-command.sh
   >   hello world
   > EOF
-  $ rt $HGTEST_RUN_TESTS_PURE test-testdir-path.t
+  $ rt test-testdir-path.t
   .
   # Ran 1 tests, 0 skipped, 0 warned, 0 failed.
 
@@ -741,7 +741,7 b' test support for --allow-slow-tests'
   >   $ echo pass
   >   pass
   > EOF
-  $ rt $HGTEST_RUN_TESTS_PURE test-very-slow-test.t
+  $ rt test-very-slow-test.t
   s
   Skipped test-very-slow-test.t: missing feature: allow slow tests
   # Ran 0 tests, 1 skipped, 0 warned, 0 failed.