refactor: remove type field, derive backend from default config

[iamazy]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 75.30864% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.31%. Comparing base (e027dd5) to head (a50453b).

Files with missing lines	Patch %	Lines
termua/src/window/new_session/actions.rs	48.27%	15 Missing ⚠️
termua/src/panel/terminal_panel.rs	80.00%	3 Missing ⚠️
crates/gpui_term/src/backends/mod.rs	0.00%	1 Missing ⚠️
termua/src/window/new_session/render.rs	93.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   55.14%   55.31%   +0.16%     
==========================================
  Files         158      159       +1     
  Lines       54854    54671     -183     
==========================================
- Hits        30249    30241       -8     
+ Misses      24605    24430     -175     

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