Revert some unnecessary, buggy pipeline additions

This reverts commit 507d7bf02768676965781d8dabf4f2098a7c57f4.

Unnesting pointer calculators was very specific to the pipelining scenario
where chunks arrive as pointer calculators and the hash table needs to find the
lowest common structure to all chunks and then recalculate the positions based
on that. For general purposes in Hyrise, this will break. If the input to
a group by is a pointer calculator, this will not work; the group by assumes
pc-relative positions whereas the hash table would be based on absolute
positions.

Removed ineffective and buggy PipelineMergeHashTables.

The implementation basically rebuilt the piped hash table chunks from scratch.
To make it actually work without rendering the hash table useless for other
operators would have required more heavy lifting than seemed reasonable for the
gained benefit. Thus, it has been abolished.

Removed useless PipeliningHashBuild.