[C++] Remove pre-C++17 compatibility measures

[asfimport]

Reporter: Antoine Pitrou / @pitrou
Assignee: Antoine Pitrou / @pitrou

Related issues:

• [C++] Remove string_view backport (is a parent of)
• [C++] Remove optional backport (is a parent of)
• [C++] Remove variant backport (is a parent of)
• [C++] Remove atomic shared_ptr compatibility functions (is a parent of)
• [C++] Remove make_unique reimplementation (is a parent of)
• [C++] Remove index_sequence_for and aligned_union backports (is a parent of)
• [C++] Remove assorted pre-C++17 compatibility measures (is a parent of)

_{Note: This issue was originally created as ARROW-17546. Please see the migration documentation for further details.}

[asfimport]

Gang Wu / @wgtmac:
Are we supposed to remove make_unique as well? @pitrou 

https://github.com/apache/arrow/blob/master/cpp/src/arrow/util/make_unique.h

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Antoine Pitrou / @pitrou:
@wgtmac We can probably do.