Removed ax_text_utils from the ax_base build target

None of the text navigation and manipulation methods in ax_text_utils.cc
were used in the ax_base build target.
The only reason that ax_text_utils was included in the above target
is for two "ToString" methods concerning default action verbs.
Since default action verbs are part of ax_enums, the appropriate place
for these two "ToString" methods is in ax_enum_util and not in ax_text_utils.
This patch moves those methods to ax_enum_util.cc and thus breaks the dependency of
ax_base on ax_text_utils.
R=dmazzoni@chromium.org, aleventhal@chromium.org

Change-Id: I781d6331f1a2ea5c985881275ce4d525410a525f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2160350
Auto-Submit: Nektarios Paisios <nektar@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762284}