/thirdparty/tinymce/plugins/googlemaps/googlemaps.php = /thirdparty/tinymce/plugins/googlemaps/googlemaps.html
You need to edit the html file so that the php code is removed and your google maps API key is added throughout the file.
//Remove
<?php $GMap_api_key = "API_KEY_HERE" ?>
//Replace
<?php echo $GMap_api_key?> //and
<?php echo $GMap_api_key ?> //note the extra space that you might miss with search and replace
//With your actual google maps API key
Lastly, if you want to hide your API key from the CMS user you can hide the API key field in:
/thirdparty/tinymce/plugins/googlemaps/googlemaps.html
<fieldset style="margin-top:10px;">
<legend>API-KEY:</legend>
<div style="padding:4px">API KEY HIDDEN<input id="akey" readonly="readonly" name="akey" type="hidden" value="YOUR_API_KEY_HERE" class="text" style="width:570px" /></div>
</fieldset>
And finally you have to open /thirdparty/tinymce/plugins/googlemaps/editor_plugin_src.js and change
(located at line 17)
file : url + '/googlemaps.php', becomes file : url + '/googlemaps.html',
Otherwise it will continue to look for the php file that no longer exists