Bug 852959 - Remove old code paths for running xpcshell tests from directories; r=ted

Manifests are now the mechanism used to specify how to run xpcshell
tests. This patch removes old code paths that allowed directories to be
specified.

runxpcshelltests.py still contains references to "testdirs." These
should likely be removed as well. This patch doesn't touch them out of
fear it may have consequences on automation.

--HG--
extra : rebase_source : 8cbfe6dc759dc0a311669c4fcba08ff7a60edc4a
extra : amend_source : 890377d9ba33d52c84e0736b0812231f3005b253