renamed a confusingly named function in cairo-font-face.c

I noticed a function called __put in cairo-font-face.c, and upon reading the commit message (337ab1f8d9e29086bfb4001508b28835b41c6390) learned that the purpose of the function was to decrement the refcount, checking if the decrement was the last one.

This commit just renames the function to something more self-explanatory and removes code that's been commented out for 8 years.