Fixed SMB rename function to overwrite target file

When uploading files through WebDAV, a part file is created and a rename
operation is performed with the expectation that the part file
overwrites an existing file, if any.

This fix makes the SMB external storage delete the target file before
renaming, as smbclient doesn't support overwrite on move/rename.

Fixes #5348 

Requires https://github.com/owncloud/core/pull/5995 to be merged first before it can be tested.

Please review the code @schiesbn @icewind1991 @DeepDiver1975 @karlitschek