Remove "Do not link against nonexistent librt (part of libc)" patch

Doing so is correct, but the patched code is under "linux* { ... }" in the
qmake file and therefore does not effect our build at all.

The resulting Makefile did not change after removing this patch, everything
built and linked fine as exepcted.

audio/mumble/patches/patch-plugins_link_link_pro

@@ -1,15 +0,0 @@
-$OpenBSD: patch-plugins_link_link_pro,v 1.5 2020/04/05 21:14:44 kn Exp $
-
-Do not link against nonexistent librt (part of libc).
-
-Index: plugins/link/link.pro
---- plugins/link/link.pro.orig
-+++ plugins/link/link.pro
-@@ -15,7 +15,4 @@ win32 {
- unix {
-   SOURCES = link-posix.cpp
- 
--  linux* {
--    LIBS          += -lrt
--  }
- }