build: remove libwebpmux from default targets/config

the extended file format is still under development and related
libs/binaries are not fit for release

configure:
--enable/disable-libwebpmux; default is disabled.

makefile.unix:
src/mux/libwebpmux.a and examples/webpmux must be explicitly specified

Makefile.vc:
$(DIRLIB)\libwebpmux.lib and $(DIRBIN)\webpmux.exe must be explicitly
specified

Change-Id: I8246746b256010dd2a2e4de58291222d7eaf0457

Android.mk

@@ -35,10 +35,6 @@ LOCAL_SRC_FILES := \
     src/enc/syntax.c \
     src/enc/tree.c \
     src/enc/webpenc.c \
-    src/mux/demux.c \
-    src/mux/muxedit.c \
-    src/mux/muxinternal.c \
-    src/mux/muxread.c \
     src/utils/bit_reader.c \
     src/utils/bit_writer.c \
     src/utils/color_cache.c \