map: remove MapSpec.Freeze field #1212

Job	Run time
Build Documentation	28s
Build and Lint	1m 56s
Run tests on arm64	1m 4s
Run tests on previous stable Go	2m 31s
Run tests (mainline)	1m 57s
Run tests (stable)	4m 50s
Run tests (6.6)	2m 14s
Run tests (6.1)	2m 29s
Run tests (5.15)	2m 16s
Run tests (5.10)	2m 14s
Run tests (5.4)	2m 7s
Run tests (4.19)	2m 14s
Run tests (4.14)	2m 22s
Run tests (4.9)	3m 49s
	32m 31s

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