mail/mu: Fix build

Fix build with MANPAGES=off and MU4E=on because BUILD_DEPENDS=emacs was
changed to MANPAGES_BUILD_DEPENDS.
Delete the *.elc files to enable compatibility with the new Emacs
framework.

PR: 295350
Approved by: rwn@mailo.com (maintainer, timeout, implicit)
Approved by: fluffy (mentor)