Mumble depends on dbus unconditionally

### Context

Mumble under Linux cannot be compiled without Qt supporting DBus. However it looks like code supports running without DBus perfectly fine and after removing check from CMakeLists.txt:

```
--- a/src/mumble/CMakeLists.txt 2025-12-27 09:41:21.077684091 +0000
+++ b/src/mumble/CMakeLists.txt 2025-12-27 09:41:34.982568210 +0000
@@ -835,19 +835,6 @@
        target_compile_definitions(mumble_client_object_lib PUBLIC "NO_UPDATE_CHECK")
 endif()

-if(NOT WIN32 AND NOT APPLE)
-       find_pkg(Qt6 COMPONENTS DBus REQUIRED)
-
-       target_sources(mumble_client_object_lib
-               PRIVATE
-                       "DBus.cpp"
-                       "DBus.h"
-       )
-
-       target_compile_definitions(mumble_client_object_lib PUBLIC "USE_DBUS")
-       target_link_libraries(mumble_client_object_lib PUBLIC Qt6::DBus)
-endif()
-
 if(translations)
        # Glob for all translation files in the source dir
        file(GLOB TS_FILES "${CMAKE_CURRENT_SOURCE_DIR}/*.ts")
```

Mumble functions perfectly fine. Please make it optional.

### Description

Have CMake flag that optionally removes dependency on DBus.

### Mumble component

Client

### OS-specific?

Yes

### Additional information

_No response_

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