Elide main queue check in Linux readpassphrase() implementation

[gwynne]

In an async world, the check for running on the main queue in linux_readpassphrase() is hopelessly wrong. Since it's a violation of the API contract to call this method from multiple threads, we just remove the bad assertion and let misuse fail out the same way the "real" API would.

Fixes #184.

[codecov-commenter]

Codecov Report

Merging #185 (d9f404f) into main (cc584bd) will decrease coverage by 0.11%.
Report is 2 commits behind head on main.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #185      +/-   ##
==========================================
- Coverage   49.01%   48.91%   -0.11%     
==========================================
  Files          45       45              
  Lines        1887     1887              
==========================================
- Hits          925      923       -2     
- Misses        962      964       +2     

Files	Coverage Δ
...ces/ConsoleKit/Terminal/readpassphrase_linux.swift	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes

[penny-for-vapor]

These changes are now available in 4.10.1