Remove WX_WEB_EXTENSIONS_DIRECTORY define

wxWidgets already has a mechanism for locating plugins under
$libdir/wx/*, there's no reason to duplicate it with another
compile-time symbol.

As a consequence, this makes the library relocatable via WXPREFIX again.

build/cmake/lib/webview/CMakeLists.txt

@@ -31,9 +31,6 @@ if(WXGTK AND wxUSE_WEBVIEW_WEBKIT2)
     else()
         set(WX_WEB_EXTENSIONS_DIRECTORY "lib/wx/${wxMAJOR_VERSION}.${wxMINOR_VERSION}/web-extensions")
     endif()
-    wx_lib_compile_definitions(wxwebview PRIVATE
-        -DWX_WEB_EXTENSIONS_DIRECTORY="${CMAKE_INSTALL_PREFIX}/${WX_WEB_EXTENSIONS_DIRECTORY}"
-        )
 endif()
 
 if(APPLE)