Remove unneeded #define in ref_counted.h

Now that the implicit conversion to T* can be removed, the preprocessor
define DISABLE_SCOPED_REFPTR_CONVERSION_OPERATOR is no longer needed.

BUG=110610

Review URL: https://codereview.chromium.org/759503002

Cr-Commit-Position: refs/heads/master@{#305591}
1 file changed