Makefile.am: remove http_caldav_js.h from nodist_imap_httpd_SOURCES

The imap/caldav_js.h and carddav_js.h files are generated by
xxd from caldav.js and carddav.js . xxd is not required as
build dependency, therefore the files caldav_js.h and
carddav_js.h must be included in the tarball on "make dist".
Mentioning the files in imap_httpd_SOURCES achieves exactly
this.

Mentioning the files in nodist_imap_httpd_SOURCES means the
opposite: do not include the files in the "make dist” tarball.
As a matter of fact, the files were included in both
nodist_imap_httpd_SOURCES and imap_httpd_SOURCES.

"make distclean" deletes files created by ./configure. Since
the caldav_js.h files are from the tarball and not generated
by ./configure, they shall not be clean by “make disclean”
but by “make maintainer-clean”.