Don't use $(RM) in src/doc/Makefile.in

We allow these rules to be used in an unconfigured tree, and some
makes do not supply a value for RM by default.

(cherry picked from commit 3f7394c1440f7526bc6f008f841b438d84411f3d)

https://github.com/krb5/krb5/commit/3a911a0d61e6ad6e178949b13174fc05ee468c4e
Author: Ben Kaduk <kaduk@mit.edu>
Committer: Tom Yu <tlyu@mit.edu>
Commit: 3a911a0d61e6ad6e178949b13174fc05ee468c4e
Branch: krb5-1.11
 src/doc/Makefile.in |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)