doc: Drop obsolete MSCGEN_PATH, PERL_PATH from doxygen configuration

doc: Drop obsolete MSCGEN_PATH, PERL_PATH from doxygen configuration

Since doxygen 1.8.16, MSCGEN_PATH and PERL_PATH are obsolete:

MSCGEN_PATH:
  https://github.com/doxygen/doxygen/commit/873e0ccfbe43ef98d4858beb08eef37631ae3cf6
PERL_PATH:
  https://github.com/doxygen/doxygen/commit/6d1535c38fe6bdaa2a00fff0e7e43774a740a4ce

I don't think that the notmuch builds ever depended on them in the
first place, and including them in the default config yields the
following two warnings:

```
doxygen ./doc/doxygen.cfg
warning: Tag 'PERL_PATH' at line 267 of file './doc/doxygen.cfg' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'MSCGEN_PATH' at line 272 of file './doc/doxygen.cfg' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
```

Remove them to avoid the warnings.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>