Revert "emacs: notmuch-search: avoid wiping out buffer-local variables"

Revert "emacs: notmuch-search: avoid wiping out buffer-local variables"

This reverts commit f9fbd1ee3bfd679175d88af403752d87a730349f.

Emacs provides a mechanism for avoiding wiping out buffer-local
variables: marking them as "permanent local", which essentially
means "don't wip out the local value when enabling major-mode".

  (put 'the-variable 'permanent-local t)

See (info "(elisp)Creating Buffer-Local").

Whether refreshing the buffer contents should involve re-enable the
mode is a different question, which should not be decided based on
the fact that we want keep the value of some random variable, not
least because some other (e.g. cache) variables are likely expected
to be wiped.