Bug 1416020 - Remove echo-variable-% target from client.mk; r=nalexander

AFAICT this is unused.

There are some mozharness consumers that evaluate an echo-variable-*
target. However, I believe they are hitting the target from debugmake.mk
and not going through client.mk.

MozReview-Commit-ID: 9XjykFMRpsT

--HG--
extra : rebase_source : edc9be971ddb0384408652fd04f550d7d8e51aa6

client.mk

@@ -286,9 +286,6 @@ build::
 	-$(MOZBUILD_MANAGE_SCCACHE_DAEMON) --stop-server
 endif
 
-echo-variable-%:
-	@echo $($*)
-
 # This makefile doesn't support parallel execution. It does pass
 # MOZ_MAKE_FLAGS to sub-make processes, so they will correctly execute
 # in parallel.