Remove unsigned type support checks in unit tests

Despite previously assumed and tested, most of the databases support
storing the various unsigned integer values. The checks for it in the
tests are therefore obsolete.
Only Firebird and SQLite seem to have a problem with storing UINT64_MAX.
The retrieval of it in a sorted result set with multiple other values
shows that the value is stored as a signed integer in the database.

tests/sqlite3/test-sqlite3.cpp

@@ -665,11 +665,6 @@ public:
         return true;
     }
 
-    bool has_full_uint64_support() const override
-    {
-        return false;
-    }
-
     bool enable_std_char_padding(soci::session&) const override
     {
         // SQLite does not support right padded char type.