Remove `tx-signer` feature (#764) #1122

Job	Run time
Test Suite (stable)	3m 59s
Test Suite (1.70.0)	4m 21s
Check	1m 45s
Build (stable)	5m 24s
Build (1.70.0)	6m 6s
Clippy	1m 48s
Rustfmt	20s
Validate against test harness	4m 1s
	27m 44s

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