Remove no-longer-necessary LiteralString promotion from solver.rs

[rchen152]

Summary: The bug that this promotion addressed is now solved more generally by partial type inference in loops.

Differential Revision: D90739317

[meta-codesync]

@rchen152 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D90739317.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scikit-learn (https://github.com/scikit-learn/scikit-learn)
+ ERROR sklearn/externals/_arff.py:587:29-47: Argument `str` is not assignable to parameter `object` with type `LiteralString` in function `list.append` [bad-argument-type]
+ ::error file=sklearn/externals/_arff.py,line=587,col=29,endLine=587,endColumn=47,title=Pyrefly bad-argument-type::Argument `str` is not assignable to parameter `object` with type `LiteralString` in function `list.append`

apprise (https://github.com/caronc/apprise)
+ ERROR apprise/plugins/sns.py:671:44-49: Argument `str | Any` is not assignable to parameter `object` with type `LiteralString` in function `list.append` [bad-argument-type]
+ ::error file=apprise/plugins/sns.py,line=671,col=44,endLine=671,endColumn=49,title=Pyrefly bad-argument-type::Argument `str | Any` is not assignable to parameter `object` with type `LiteralString` in function `list.append`

werkzeug (https://github.com/pallets/werkzeug)
+ ERROR src/werkzeug/debug/tbtools.py:418:17-420:50: Argument `str` is not assignable to parameter `object` with type `LiteralString` in function `list.append` [bad-argument-type]
+ ::error file=src/werkzeug/debug/tbtools.py,line=418,col=17,endLine=420,endColumn=50,title=Pyrefly bad-argument-type::Argument `str` is not assignable to parameter `object` with type `LiteralString` in function `list.append`

[stroxler]

Review automatically exported from Phabricator review in Meta.