Remove inline styling from Notification RSS Service

AFFECTS:

• xwiki:XWiki.Notifications.Code.XWikiUserNotificationsSheet

ERROR:

• org.xwiki.test.webstandards.CustomDutchWebGuidelinesValidationTest.Validating HTML5 Dutch Web Guidelines validity (from org.xwiki.test.webstandards.AllTests)
• ERROR: At least one style attribute has been found on the page.

INVALIDATES:

• Dutch Guidelines: validateRpd9s1()
  • https://github.com/xwiki/xwiki-platform/blob/master/xwiki-platform-tools/xwiki-platform-tool-standards-validator/src/main/java/org/xwiki/validator/DutchWebGuidelinesValidator.java#L1013
• R-pd.9.1 "CSS should be placed in linked files and not mixed with the HTML source code."