Remove `tx-signer` feature #1121

Job	Run time
Check	1m 43s
Clippy	2m 26s
Build (stable)	7m 41s
Rustfmt	17s
Build (1.70.0)	7m 7s
Test Suite (stable)	4m 4s
Validate against test harness	3m 31s
Test Suite (1.70.0)	5m 17s
	32m 6s

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