Remove fdelt_chk back-compat code and sanity check

Summary:

The return type of fdelt_chk changed from unsigned long int to long
int in glibc 2.16. See this commit. Now that we require glibc >=2.17 we
can remove our back-compat code.
[...]

Note: we require glibc >= 2.19 so this also applies to us.

Backport of core PR15146 and PR18862.
PR15146 fix an edge case from the sanity check which is totally removed
by the second commit of PR18862. The diff is then mostly a backport of
the first commit from PR18862:
https://github.com/bitcoin/bitcoin/pull/18862/commits/8bf1540cc235fb8fb5330a7ae8ab638247ceb177

Test Plan: Run the gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6342