Don't check . ($PWD) when searching for something in the GeoServer data dir

Our app server runs with a present working directory of /, and there is a directory named /security. GeoServerResourceLoader.find(File, File) calls File.exists() even on relative paths, effectively searching $PWD for data dir entries. If GeoServer needs to search $PWD, and I don't think it should do so implicitly, it should do so after GeoServerResourceLoader.searchLocations is searched, or the startup code could just add "." to the end of GeoServerResourceLoader.searchLocations after GEOSERVER_DATA_DIR and the other usual search paths. This pull request just removes the check for File.exists().