[MRG] Remove deprecated ot.gpu submodule

[rflamary]

Types of changes

Motivation and context / Related issue

How has this been tested (if it applies)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

Merging #361 (3debbd1) into master (ad02112) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #361   +/-   ##
=======================================
  Coverage   93.75%   93.75%           
=======================================
  Files          23       23           
  Lines        5845     5845           
=======================================
  Hits         5480     5480           
  Misses        365      365