[utility.syn, flat.map.defn] Remove all [[nodiscard]] from library wording

[Mick235711]

After extensive discussion about [[nodiscard]] policy in LEWG, it was agreed that [[nodiscard]] should not appear in the library wording (see SD-9). However, the wording removal paper, P2422R1 (approved in St. Louis, #7110), does not seem to remove all occurrences of [[nodiscard]] in the library clauses due to some recently added [[nodiscard]] annotation (such as those on flat containers’ empty()) being missed.

Therefore, this PR removes all occurrences of [[nodiscard]] in N4988. Those are:

• 22.2.1 [utility.syn]
• 24.6.9.2 [flat.map.defn]
• 24.6.10.2 [flat.multimap.defn]
• 24.6.11.2 [flat.set.defn]
• 24.6.12.2 [flat.multiset.defn]
• 24.7.3.6.1 [mdspan.mdspan.overview] and 24.7.3.6.3 [mdspan.mdspan.members]

After the change, the only mention of [[nodiscard]] in the standard is in examples and Core wording.

(I assumed that this is editorial since it seems likely that these are just missed by P2422's wording...)

[jensmaurer]

@jwakely , this seems like an editorially-obvious sync operation. What do you think?