Remove the logic for skipping existing processing

[stephen-riggs]

When we had the TUI we introduced logic for skipping processing in the case that someone started a fresh murfey with existing data. This no longer makes any sense as murfey can handle switching between tomo and SPA without a fresh instance.

Therefore this PR removes all skip_existing_processing and first_loop logic.

The case that needs checking is if processing is disabled, then it should not become enabled on reconnection. From the code this loops ok to me as the start_multigrid_watcher requests to the instrument server read session.process from the database.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.66%. Comparing base (0df33b0) to head (3b35bcd).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #735      +/-   ##
==========================================
+ Coverage   46.59%   46.66%   +0.06%     
==========================================
  Files          91       91              
  Lines        9634     9611      -23     
  Branches     1262     1256       -6     
==========================================
- Hits         4489     4485       -4     
+ Misses       4932     4913      -19     
  Partials      213      213              

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