sbr/mf.c: Remove unused functions: isfrom, lequal, mfgets.

sbr/mf.c: Remove unused functions: isfrom, lequal, mfgets.

isfrom(s) checked for /^>?From /.
lequal(a, b) was another case-insensitive strcmp(3).
mfgets(fp, line) fetched a header at a time from fp, indicating when the
end of headers was reached.
Investigating their historical usage was, ironically, made too tedious
to complete by the pollution of mass commits of historical MH.