popen: remove OOM handling code

Allocation from malloc and the fiber region never fail so we can use x*
variants to simplify code and improve coverage.

NO_TEST=code cleanup
NO_CHANGELOG=code cleanup

@TarantoolBot document
Title: Remove `OutOfMemory` error from `popen.new` and `popen.read`

These functions don't raise the `OutOfMemory` exception anymore so all
mentions of it should be removed from the documentation:

 - https://www.tarantool.io/en/doc/latest/reference/reference_lua/popen/#popen-new
 - https://www.tarantool.io/en/doc/latest/reference/reference_lua/popen/#popen-read