Drop '-Wno-shadow' previously set for qt5/kde5

The flag has been introduced with commit
9335945c7c ("KF5 initial VCL plugin")
and was presumably just copy-pasted in commit
4d78cf97d7.

The flag should no longer be necessary now; the only issue showing
up after removing the flag was fixed.

Change-Id: I65fe4d12bdf7d5888b75f8a5743bab6979a63295
Reviewed-on: https://gerrit.libreoffice.org/73892
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

RepositoryExternal.mk

@@ -3092,12 +3092,6 @@ $(call gb_LinkTarget_add_libs,$(1),\
 	$(KF5_LIBS) \
 )
 
-ifeq ($(COM),GCC)
-$(call gb_LinkTarget_add_cxxflags,$(1),\
-	-Wno-shadow \
-)
-endif
-
 endef
 
 else # !ENABLE_KDE5
@@ -3126,12 +3120,6 @@ $(call gb_LinkTarget_add_libs,$(1),\
 	$(QT5_LIBS) \
 )
 
-ifeq ($(COM),GCC)
-$(call gb_LinkTarget_add_cxxflags,$(1),\
-       -Wno-shadow \
-)
-endif
-
 endef
 
 else # !ENABLE_QT5