BUG/MEDIUM: sample: simplify and fix the argument parsing

BUG/MEDIUM: sample: simplify and fix the argument parsing

Some errors may be reported about missing mandatory arguments when some
sample fetch arguments are marked as mandatory and implicit (eg: proxy
names such as in table_cnt or be_conn).

In practice the argument parser already handles all the situations very
well, it's just that the sample fetch parser want to go beyond its role
and starts some controls that it should not do. Simply removing these
useless controls lets make_arg_list() create the correct argument types
when such types are encountered.

This regression was introduced by the recent use of sample_parse_expr()
in ACLs which makes use of its own argument parser, while previously
the arguments were parsed in the ACL function itself. No backport is
needed.