Revert "BUG io: fix file descriptor closing for netcdf variables"

This reverts commit 2dfffc7beeb924d6aa5e6529f25745c5330a7bac.

Closing mmap fd's can cause segfaults, as discussed in gh-3630
Revert "TST: add regression test for gh-1550 (netcdf_file leaves mmaps open)."

This reverts commit e24f1e1e218ef683fe43b1a7143369822b61ccf7.
Merge pull request #3961 from pv/netcdf-fix-0.14.x

Revert segfault-causing netcdf changes in 0.14.x