Remove undeclared unused extension_ime_util::IsMemberOfExtension().

Should've been removed together in https://crrev.com/c/2437720 but was
accidentally missed out...

Bug: 1134465
Change-Id: I7d338c87cd1081fc1b3ae5629b2a4db5ce4422f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568339
Auto-Submit: Bao-Duy Tran <tranbaoduy@chromium.org>
Commit-Queue: Keith Lee <keithlee@chromium.org>
Reviewed-by: Keith Lee <keithlee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834092}

ui/base/ime/chromeos/extension_ime_util.cc

@@ -159,13 +159,6 @@ bool IsArcIME(const std::string& input_method_id) {
          input_method_id.size() > kArcIMEPrefixLength + kExtensionIdLength;
 }
 
-bool IsMemberOfExtension(const std::string& input_method_id,
-                         const std::string& extension_id) {
-  return base::StartsWith(input_method_id,
-                          kExtensionIMEPrefix + extension_id,
-                          base::CompareCase::SENSITIVE);
-}
-
 bool IsKeyboardLayoutExtension(const std::string& input_method_id) {
   if (IsComponentExtensionIME(input_method_id))
     return base::StartsWith(GetComponentIDByInputMethodID(input_method_id),