Lintian fix: remove unnecessary Priority: optional definition

The entire source package has Priority: optional so it is useless for
any package to redefine that.

debian/control

@@ -62,7 +62,6 @@ Package: libmariadb-dev-compat
 Architecture: any
 Multi-Arch: same
 Section: libdevel
-Priority: optional
 Depends: libmariadb-dev (= ${binary:Version}),
          ${misc:Depends}
 Conflicts: libmariadb-client-lgpl-dev-compat,