remotefilelog: drop compat code for "getbundle_shallow" wireprotocol command
Doing some annotate on hgexperimental shows that getbundle_shallow used to exist
in 2013 or before. We don't have any pre-2013 remotefilelog users except Fb
themselves and I doubt they are going to use in-core remotefilelog. So it's safe
to remove this.
Differential Revision: https://phab.mercurial-scm.org/D5193
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