MAINT: replace deprecated version check for traitsui (#1746)

In traitsui, traitsui.__version__ has been deprecated and should be
replaced with importlib.metadata.version('traitsui'). The deprecation
warning raised have also caused test_edit_not_given in
traits/traits/tests/test_configure_traits.py to fail as mentioned in
#1745 . Since the version check is a temporary check to address the
dependency of traits on traits ui and is no longer need, the current PR
fixed this issue by removing all version checks related to
_traitsui_helpers.py.

Closes #1745

Co-authored-by: Chengyu Liu <cyliu@aus552cyliu.local>

traits/has_traits.py

@@ -55,7 +55,6 @@
 )
 from .trait_errors import TraitError
 from .util.deprecated import deprecated
-from .util._traitsui_helpers import check_traitsui_major_version
 from .trait_converters import check_trait, mapped_trait_for, trait_for
 
 
@@ -1846,10 +1845,6 @@ def _trait_view(
         if view_elements is None:
             return None
 
-        # Provide a nicer failure mode when upgrading to Traits using
-        # TraitsUI 6.x
-        check_traitsui_major_version(7)
-
         if name:
             if view_element is None:
                 # If only a name was specified, return the ViewElement it