message: remove CFG_RELEASE from the Makefile
  The previous commit made this unused; much of the logic for assigning to
  this variable now lives in Cargo itself.
diff: @@ -9,16 +9,12 @@ include config.mk
  export PATH := $(dir $(CFG_RUSTC)):$(PATH)
  ifeq ($(CFG_RELEASE_CHANNEL),stable)
  -CFG_RELEASE=$(CFG_RELEASE_NUM)
  CFG_PACKAGE_VERS=$(CFG_RELEASE_NUM)
  else ifeq ($(CFG_RELEASE_CHANNEL),beta)
  -CFG_RELEASE=$(CFG_RELEASE_NUM)-beta$(CFG_PRERELEASE_VERSION)
  CFG_PACKAGE_VERS=beta
  else ifeq ($(CFG_RELEASE_CHANNEL),nightly)
  -CFG_RELEASE=$(CFG_RELEASE_NUM)-nightly
  CFG_PACKAGE_VERS=nightly
  else ifeq ($(CFG_RELEASE_CHANNEL),dev)
  -CFG_RELEASE=$(CFG_RELEASE_NUM)-dev
  CFG_PACKAGE_VERS=$(CFG_RELEASE_NUM)-dev
  endif
  