Remove http_block, block_cache, and test/block

[nfeske]

Those components rely on the old block/component.h framework classes, which have long been superseded by the Block::Request_stream. In order to dispose the legacy block/component.h API, those components would need to be updated. However, given that they are lingering unused since years, I doubt that the effort is justified.

Should the functionality become needed, we would probably want to implement those features differently, anyway. E.g., supporting SSL for accessing a block device over HTTP, or implementing a block cache as VFS plugin instead of a bump-in-the-wire server. So I don't see a revival of the existing components very likely.

I presume that test/block has been superseded by app/block_tester and vfs_block in the meantime.

@skalk and @cnuke, what is your opinion?

[cnuke]

@nfeske I am fine with removing them.

[nfeske]

Fixed in master.