Fix minor makefile issues

Summary:
@public
- Remove `No such file or directory` unrelated errors when building Infer for Java only:

  line 0: cd: infer/../facebook-clang-plugin: No such file or directory

- Remove makefile comments from stdout

Test Plan:
Ran this command without any checkout of `facebook-clang-plugins`
  make -C infer java
The misleading error messages are gone.

infer/src/Makefile

@@ -86,8 +86,7 @@ CLANG_SOURCES = clang
 INFERCLANG_MAIN = $(CLANG_SOURCES)/cMain
 INFERCLANG_BINARY = $(BINDIR)/InferClang
 
-CLANG_PLUGIN_ROOT ?= $(shell cd $(ROOT)/../facebook-clang-plugin && pwd)
+CLANG_PLUGIN_ROOT ?= $(ROOT)/../facebook-clang-plugin
-CLANG_PLUGIN_BUILD = $(CLANG_PLUGIN_ROOT)/build
 CLANG_PLUGIN_BINARIES = $(addprefix $(CLANG_PLUGIN_ROOT)/clang-ocaml/build/, clang_ast_converter clang_ast_named_decl_printer)
 
 CLANG_OCAML_ROOT = $(CLANG_PLUGIN_ROOT)/clang-ocaml