test: Remove useless NOTMUCH variable (in favor of simply "notmuch")

test: Remove useless NOTMUCH variable (in favor of simply "notmuch")

When the NOTMUCH variable was originally invented it was used as an
explicit path to the notmuch binary being tested. Today, the test
suite sets the PATH variable instead, so the NOTMUCH variable always
has a value of simply "notmuch".

We simplifying that by using the constant value rather than the
continual variable reference.