Global: make FFinstance instance a global variable

As discussed in https://github.com/fastfetch-cli/fastfetch/discussions/473

src/detection/datetime/datetime.c

@@ -4,10 +4,8 @@
 
 #include <time.h>
 
-const FFDateTimeResult* ffDetectDateTime(const FFinstance* instance)
+const FFDateTimeResult* ffDetectDateTime(void)
 {
-    FF_UNUSED(instance); //We may need it later for additional configuration
-
     static FFDateTimeResult result;
     static FFThreadMutex mutex = FF_THREAD_MUTEX_INITIALIZER;
     static bool init = false;

src/detection/datetime/datetime.h

@@ -30,6 +30,6 @@ typedef struct FFDateTimeResult
     FFstrbuf secondPretty; //37
 } FFDateTimeResult;
 
-const FFDateTimeResult* ffDetectDateTime(const FFinstance* instance);
+const FFDateTimeResult* ffDetectDateTime();
 
 #endif