[C++] Map type should always use "key"/"value"/"entries" for field names

[kou]

Describe the bug, including details regarding any error messages, version, and platform.

#49415 / #49416 changed to use field names in FlatBuffers for map type but it's not correct change.

Our specification defines "key"/"value"/"entries" for field names:

arrow/format/Schema.fbs

Lines 127 to 130 in 8e625d0

	/// In a field with Map type, the field has a child Struct field, which then
	/// has two children: the key type and the value type. The names of the
	/// child fields may be respectively "entries", "key", and "value", but this is
	/// not enforced.

We can use other field names but using the standard field names are better.

So we should revert #49416.

Component(s)

C++

[raulcd]

Issue resolved by pull request 49457
#49457