Drop redundant find_package(Qt5Multimedia)

We need the multimedia component only to play preview videos in a KCM.

The find_package(Qt5Multimedia) is not needed because we already check
if Qt5Multimedia QML module is present using ecm_find_qmlmodule().

The main motivation behind this change is to work around constant build
failures caused by buggy config file of the Qt5::Multimedia component.

CMakeLists.txt

@@ -59,12 +59,6 @@ add_definitions(-DEGL_NO_PLATFORM_SPECIFIC_TYPES)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
-find_package(Qt5Multimedia QUIET)
-set_package_properties(Qt5Multimedia PROPERTIES
-    PURPOSE "Runtime-only dependency for effect video playback"
-    TYPE RUNTIME
-)
-
 # required frameworks by Core
 find_package(KF5 ${KF5_MIN_VERSION} REQUIRED COMPONENTS
     Config