Remove file-name completion in Madx.call

IMO, files should always be specified by their full name. Trying to guess a
different name should not be the responsibility of pymad. For example, the
current implementation doesn't care if there are .str and .madx files with
the same base name, which can lead to indeterministic program behaviour -
consider the following case:

- call('some-file')  ->  calls some-file.str
- some-file.madx is added on the file-system
- call('some-file')  ->  calls some-file.madx