Remove unused, unimplemented and no-op methods from MIMETypeRegistry.

The following methods have been removed.

Unimplemented and unused:
MIMETypeRegistry::getExtensionForMIMEType(const String&)
MIMETypeRegistry::getMediaMIMETypesForExtension(const String&)
MIMETypeRegistry::isUnsupportedTextMIMEType(const String&)
MIMETypeRegistry::canShowMIMEType(const String&)
MIMETypeRegistry::getUnsupportedTextMIMETypes()
MIMETypeRegistry::getNormalizedMIMEType(const String&)
defaultMIMEType()

Trivial and unused:
MIMETypeRegistry::getSupportedImageMIMETypes()
MIMETypeRegistry::getSupportedImageResourceMIMETypes()
MIMETypeRegistry::getSupportedImageMIMETypesForEncoding()
MIMETypeRegistry::getSupportedNonImageMIMETypes()
MIMETypeRegistry::getSupportedMediaMIMETypes()

Trivial implementations:
MIMETypeRegistry::getMediaMIMETypeForExtension(const String&)
MIMETypeRegistry::isApplicationPluginMIMEType(const String&)

BUG=268948

Review URL: https://chromiumcodereview.appspot.com/21929002

git-svn-id: svn://svn.chromium.org/blink/trunk@155631 bbb929c8-8fbe-4397-9dbb-9b2b20218538