Raise libkomparediff2 version requirement to 5.1

This allows us to unconditionally compile with the strict string
conversion rules. I cannot find an exact time when this got released
but it's been regularly released with higher version numbers for many
years by now.

plugins/patchreview/CMakeLists.txt

-find_package(LibKompareDiff2 5.0 CONFIG)
+find_package(LibKompareDiff2 5.1 CONFIG)
 set_package_properties(LibKompareDiff2 PROPERTIES
     PURPOSE "Required for building the patch review plugin."
     TYPE REQUIRED
@@ -19,13 +19,6 @@ endif()
 add_definitions(-DTRANSLATION_DOMAIN=\"kdevpatchreview\")
 kde_enable_exceptions()
 
-if(LibKompareDiff2_VERSION VERSION_LESS 5.1)
-    remove_definitions(
-        -DQT_NO_CAST_FROM_ASCII
-        -DQT_NO_SIGNALS_SLOTS_KEYWORDS
-    )
-endif()
-
 set(patchreview_PART_SRCS
     patchreview.cpp
     patchhighlighter.cpp