build: Fix gwt-debug target in Makefile

Currently, gwt-debug target executes following command:

  mvn $(DEV_BUILD_FLAGS) ...

This means DEV_BUILD_FLAGS can apply parameters that aren't
relevant and/or shouldn't even be specified when launching
GWT Developer Mode, such as:
  -Pgwtdraft [via DEV_BUILD_GWT_DRAFT]
  -Dgwt.userAgent [via DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS]

Since DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS defaults to using
Firefox (-Dgwt.userAgent=gecko1_8), if someone wants to
debug GWT application in Chrome (or any other browser), he
must override DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS with empty
string, for example:

  make gwt-debug webadmin \
    DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS=""

In other words, gwt-debug target shouldn't apply GWT-compile
time parameters, such as -Dgwt.userAgent=...

Change-Id: I333c3552807e76c722056231a9dedb039c81bdbb
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>