Remove variable time computations

[tlepoint]

It seems that after #238 , there is no more advantage of doing variable time computations, even on Apple M1/M2(!). This PR would remove all the "variable time" consideration from all the crates.

It requires full benchmarking on multiple platforms before merge.

[codecov[bot]]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (7be5ece) 93.71% compared to head (f991200) 93.81%.

Files	Patch %	Lines
crates/fhe/src/bfv/keys/key_switching_key.rs	81.81%	2 Missing :warning:
crates/fhe/src/bfv/keys/relinearization_key.rs	50.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #245      +/-   ##
==========================================
+ Coverage   93.71%   93.81%   +0.09%     
==========================================
  Files          42       42              
  Lines        8194     7742     -452     
==========================================
- Hits         7679     7263     -416     
+ Misses        515      479      -36     

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