[Build System: build-presets] Remove duplicate flag from preset which was causing the build_swift tests to fail when run with a Python 3 interpreter.

Author: Ross Bayer

utils/build-presets.ini

@@ -533,8 +533,6 @@ build-subdir=buildbot_incremental
 release
 assertions
 
-build-swift-stdlib-unittest-extra
-
 # We run the OS X tests and validation tests.
 test
 validation-test