Revert "Feature: Add SHA256 for PoolByteArray"

[akien-mga]

This reverts commit e2c3bba from #24269.

This was superseded by #29871 which adds more crypto features with a
dedicated interface.

Since this commit was never in a stable release (merged during 3.2 dev),
we revert it to avoid having to deprecate it in favor of the HashingContext API.
See #31187 (comment)

@xsellier FYI

[akien-mga]

Example from #29871:

extends Node

func _ready():
	var ctx = HashingContext.new()
	ctx.start(HashingContext.HASH_SHA256) # Or HASH_SHA1 or HASH_MD5
	print("a".to_utf8().size()) # 1
	ctx.update("a".to_ascii())
	var fin = ctx.finish()
	print(Array(fin)) # [202, 151, 129, 18, 202, 27, 189, 202, 250, 194, 49, 179, 154, 35, 220, 77, 167, 134, 239, 248, 20, 124, 78, 114, 185, 128, 119, 133, 175, 238, 72, 187]
	print(fin.hex_encode()) # ca978112ca1bbdcafac231b39a23dc4da786eff8147c4e72b9807785afee48bb