Remove `FormatFStringPart` #35931

Job	Run time
cargo fmt	16s
Determine changes	9s
pre-commit	39s
python package	1m 18s
mkdocs	2m 52s
cargo test (linux)	4m 7s
cargo test (wasm)	1m 28s
cargo clippy	2m 2s
cargo test (windows)	8m 33s
cargo build (release)	5m 15s
cargo build (msrv)	4m 6s
cargo fuzz	5m 10s
cargo shear	25s
test scripts	1m 1s
formatter instabilities and black similarity	5m 35s
benchmarks	4m 43s
test ruff-lsp	43s
ecosystem	6m 1s
Fuzz the parser	0s
	54m 23s

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