Remove empty .gitmodules

[romanc]

Description

A file called .gitmodules is usually used to store informations about git subodules. It appears that pyFMS has no submodules. It is thus unucessary - and imo confusing - to keep an empty .gitmodules file around.

This is a follow-up from #39, which moved the cFMS dependency from a submodule to be pulled via CMakeList.txt.

How Has This Been Tested?

If pyFMS was relying on submodules to be installed (it is not), CI would fail.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas: N/A
• I have made corresponding changes to the documentation: N/A
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules: N/A
• New check tests, if applicable, are included: N/A