[scm] clean up git-commands methods declared as properties

[akosyakov]

It makes sense only if they are used as React callbacks which is not the case.

cc @vinokurig

[vinokurig]

@akosyakov Could you please clarify what is the problem, is it related to #5256?

[akosyakov]

It's minor. You don't need to look into it immediately.

Here for example:
/~https://github.com/theia-ide/theia/blob/89f71edc4b417b8ce717897018c2aa1210435cd4/packages/git/src/browser/git-commands.ts#L166-L169

openFile method is declared as a property, it is not necessary, it can be just a method. Such pattern should be only used to pass functions by a reference into react event handlers to improve performance.