Stop exporting HOMEBREW_VERBOSE to build environment

This flag no longer affects the output of the compiler wrappers, since
everything is buffered by the build process rather than each individual
tool.

Additionally, this is a user-facing knob that affects ARGV.verbose?, and
the combination of these two issues broke the --quieter feature.

Library/Homebrew/extend/ENV/super.rb

@@ -77,7 +77,6 @@ module Superenv
     self['HOMEBREW_TEMP'] = HOMEBREW_TEMP
     self['HOMEBREW_SDKROOT'] = "#{MacOS.sdk_path}" if MacOS::Xcode.without_clt?
     self['HOMEBREW_DEVELOPER_DIR'] = determine_developer_dir # used by our xcrun shim
-    self['HOMEBREW_VERBOSE'] = "1" if ARGV.verbose?
     self['HOMEBREW_OPTFLAGS'] = determine_optflags
     self['CMAKE_PREFIX_PATH'] = determine_cmake_prefix_path
     self['CMAKE_FRAMEWORK_PATH'] = determine_cmake_frameworks_path