PVF validation host: do not alter niceness (#4525)

We wanted to change niceness to accomodate the fact that some of the
preparation tasks are low priority. For example, when a node sees that
there is a new para was onboarded the node may start preparing right
away. Since all other activities are more important, such as network I/O
or validation of the backed candidates and preparation of the
immediatelly needed PVFs.

However, it turned out that this approach does not work: generally
non-root processes can only decrease niceness and they cannot increase
it to the previous value, as was assumed by the code.

Apart from that, https://github.com/paritytech/polkadot/pull/4123
assumes all PVFs are prepared in the same way. Specifically, that if a
PVF preparation failed before, then PVF pre-checking will also report
that it was failed, even though it could happen that preparation failed
due to being low-priority. In order to avoid such cases, we decided to
simplify the whole preparation model. Preparation under low priority
does not work well with that.

Closes https://github.com/paritytech/polkadot/issues/4520