Remove `SYNAPSE_USE_FROZEN_DICTS` environment variable #4561

Job	Run time
changes	3s
changes	3s
lint-crlf	3s
lint-crlf	3s
check-lockfile	4s
check-lockfile	4s
lint-newsfile	35s
lint-newsfile	35s
lint	18s
lint	18s
Typechecking	1m 34s
Typechecking	1m 34s
check-schema-delta	47s
check-schema-delta	47s
check-sampleconfig	43s
check-sampleconfig	43s
lint-pydantic	1m 0s
lint-pydantic	1m 0s
lint-clippy	0s
lint-clippy	0s
lint-clippy-nightly	0s
lint-clippy-nightly	0s
lint-readme	0s
lint-readme	0s
lint-rustfmt	0s
lint-rustfmt	0s
linting-done	2s
linting-done	2s
calculate-test-jobs	5s
calculate-test-jobs	5s
trial-olddeps	6m 59s
trial-olddeps	6m 59s
portdb (3.9, 13)	3m 27s
portdb (3.9, 13)	3m 27s
complement (monolith, SQLite)	13m 29s
complement (monolith, SQLite)	13m 29s
trial-pypy	0s
trial-pypy	0s
portdb (3.13, 17)	3m 26s
portdb (3.13, 17)	3m 26s
complement (monolith, Postgres)	14m 40s
complement (monolith, Postgres)	14m 40s
complement (workers, Postgres)	22m 4s
complement (workers, Postgres)	22m 4s
cargo-bench	0s
cargo-bench	0s
cargo-test	0s
cargo-test	0s
sytest (bullseye)	8m 27s
sytest (bullseye)	8m 27s
trial (3.9, sqlite, all)	5m 59s
trial (3.9, sqlite, all)	5m 59s
sytest (bullseye, postgres)	10m 30s
sytest (bullseye, postgres)	10m 30s
trial (3.9, postgres, 13, all)	13m 28s
trial (3.9, postgres, 13, all)	13m 28s
sytest (bullseye, multi-postgres, workers)	13m 30s
sytest (bullseye, multi-postgres, workers)	13m 30s
trial (3.9, sqlite)	5m 45s
trial (3.9, sqlite)	5m 45s
sytest (bullseye, multi-postgres, workers, asyncio)	19m 29s
sytest (bullseye, multi-postgres, workers, asyncio)	19m 5s
export-data	2m 16s
export-data	2m 16s
tests-done	2s
tests-done	4s
	4h 57m 8s

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