Remove deprecated apply snapshot method

[JaySon-Huang]

Enhancement

Before v5.1, we apply snapshot by decoding SSTFiles into blocks and flush to the storage layer. Since 5.1, by default we decode SSTFiles into DTFiles and ingest to the storage layer, which consume less memory. #1347

We can remove related code of SnapshotApplyMethod::Block now.

tiflash/etc/config-template.toml

Lines 133 to 136 in 0f56b15

	[raft.snapshot]
	# The way to apply snapshot data
	# The value is one of "block" / "file1"
	# method = "file1"

[hongyunyan]

/assign hongyunyan