build: remove warning suppression

This is to track https://www.nsnam.org/bugzilla/show_bug.cgi?id=2936

The bug states that -Wno-parentheses is needed to overcome a bug in GTK3 which cause gcc >= 8 to fail compilation due to parenthesis mismatch in GTK headers. The GTK was supposedly fixed starting from 3.22 - it turns out that it's fixed in 3.24.

Unfortunately, GTL+3.24 is not available for Ubuntu 18.04 and some other distributions (see #339 (closed)).

Hence, the -Wno-parentheses workaround have to be kept in src/config-store/wscript until we phase out these older distributions.