include/dg/ADT/NumberSet.h: Fix '-Wdefaulted-function-deleted'
include/dg/ADT/NumberSet.h: Fix '-Wdefaulted-function-deleted'

As the assignment operator is never used in dg's codebase,
it's same to remove it.

Fixes:
[ 13%] Building CXX object tests/CMakeFiles/disjunctive-intervals-map-test.dir/disjunctive-intervals-map-test.cpp.o
In file included from /dg/tests/numbers-set-test.cpp:4:
/dg/include/dg/ADT/NumberSet.h:132:25: warning: explicitly defaulted copy assignment operator is implicitly deleted [-Wdefaulted-function-deleted]
        const_iterator& operator=(const const_iterator&) = default;
                        ^
/dg/include/dg/ADT/NumberSet.h:106:20: note: copy assignment operator of 'const_iterator' is implicitly deleted because field 'is_small' is of const-qualified type 'const bool'
        const bool is_small;
                   ^

mchalupa/dg,mchalupa/dg,mchalupa/dg,mchalupa/dg