[BUGFIX] Remove extension setting using deleted userFunc

The userFunc was removed with dc00dbc1.
It used internal TYPO3 functions to display a simple list of the
available mail templates, so deleting the setting entirely should have no negative impact.

ext_conf_template.txt

@@ -4,8 +4,5 @@ excludeTemplatesAllDomains =
 # cat=general; type=string; label=LLL:EXT:sg_mail/Resources/Private/Language/locallang.xlf:configuration.excludeTemplates
 excludeTemplates =
 
-# cat=general; type=user[SGalinski\SgMail\UserFunc\ExtensionConfiguration->displayTemplates]; label=LLL:EXT:sg_mail/Resources/Private/Language/locallang.xlf:configuration.templateList
-templates =
-
 # cat=general; type=boolean; label=LLL:EXT:sg_mail/Resources/Private/Language/locallang.xlf:configuration.enableSecurity
 supportOldRegistrations = 0