EPUB export: fix gla11y warnings

Change-Id: I4d2ca93ff01e27ee3400adab230cb8679aacdaa9
Reviewed-on: https://gerrit.libreoffice.org/54727Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

solenv/sanitizers/ui/writerperfect.suppr

-writerperfect/uiconfig/ui/exportepub.ui://GtkComboBoxText[@id='versionlb'] duplicate-mnemonic

writerperfect/uiconfig/ui/exportepub.ui

@@ -90,7 +90,6 @@
                     <property name="margin_top">6</property>
                     <property name="label" translatable="yes" context="exportepub|generalft">General</property>
                     <property name="use_underline">True</property>
-                    <property name="mnemonic_widget">versionlb</property>
                     <property name="xalign">0</property>
                     <attributes>
                       <attribute name="weight" value="bold"/>
@@ -428,7 +427,6 @@
                     <property name="margin_top">6</property>
                     <property name="label" translatable="yes" context="exportepub|metadataft">Metadata</property>
                     <property name="use_underline">True</property>
-                    <property name="mnemonic_widget">versionlb</property>
                     <property name="xalign">0</property>
                     <attributes>
                       <attribute name="weight" value="bold"/>