build: remove IMHEX_PLUGINS_IN_SHARE option + only allow AppImage to load plugins from inside itself

Rationale: The `IMHEX_PLUGINS_IN_SHARE` is a hack to prevent the appimage from loading plugin from system imhex installation, like /usr/lib/imhex/

In reality, I do not think people compile plugins specifically for the AppImage (plugins must be compiled for the specific imhex & compiler version the imhex binary is used), and this lets us remove the hack

lib/libimhex/source/helpers/default_paths.cpp

@@ -97,6 +97,13 @@ namespace hex::paths {
     }
 
     static std::vector<std::fs::path> getPluginPaths() {
+        // If running from an AppImage, only allow loaded plugins from inside it
+        #if defined(OS_LINUX)
+        if(const char* appdir = std::getenv("APPDIR")) { // check for AppImage environment
+            return {std::string(appdir) + "/usr/lib/imhex"};
+        }
+        #endif
+
         std::vector<std::fs::path> paths = getDataPaths(true);
 
         // Add the system plugin directory to the path if one was provided at compile time