[Extensions] Quick removal of unused method declaration.

Bug: None
Change-Id: I91bb493036a66daa99da8932e167ef2e22dd805d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2505882Reviewed-by: David Bertoni <dbertoni@chromium.org>
Commit-Queue: Solomon Kinard <solomonkinard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822060}

extensions/common/extension.h

@@ -377,8 +377,6 @@ class Extension : public base::RefCountedThreadSafe<Extension> {
   bool LoadManifestVersion(base::string16* error);
   bool LoadShortName(base::string16* error);
 
-  bool CheckMinimumChromeVersion(base::string16* error) const;
-
   // The extension's human-readable name. Name is used for display purpose. It
   // might be wrapped with unicode bidi control characters so that it is
   // displayed correctly in RTL context.