cmdutil: remove redundant commitfunc parameter in amend (API)
Since the redundant commit during the amend has been been removed, there is no
need for commit callback function in amend now. Therefore, this commit removes
the unused parameter "commmitfunc" which was being used for this purpose.
Test Plan:
Ensured that all the tests pass
Differential Revision: https://phab.mercurial-scm.org/D635
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