vcs-svn: remove header files as well

fc47391 (drop vcs-svn experiment, 2020-08-13) removed most vcs-svn
files.  Drop the remaining header files as well, as they are no longer
used.

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe