[C++] Remove deprecated APIs from v16-v18 releases

[AliRana30]

Describe the enhancement requested

Remove C++ APIs deprecated in releases v16.0.0, v17.0.0, and v18.0.0 (all before January 2025):

1. decimal(precision, scale) - deprecated in v18.0.0 (Sept 30, 2024, PR GH-43956: [C++][Format] Add initial Decimal32/Decimal64 implementations #43957)

   • Replacement: smallest_decimal()

2. cuda::DefaultMemoryMapper(device_type, device_id) - deprecated in v16.0.0 (April 16, 2024, PR GH-40698: [C++] Create registry for Devices to map DeviceType to MemoryManager in C Device Data import #40699)

   • Replacement: arrow::DefaultDeviceMapper

3. HasValidityBitmap(Type::type id) - deprecated in v17.0.0 (July 11, 2024, PR GH-41114: [C++] Add is_validity_defined_by_bitmap() predicate #41115)

   • Replacement: may_have_validity_bitmap()

This is part of the broader cleanup tracked in #49153, but focuses only on these specific C++ deprecations.

Component(s)

C++

[kou]

Issue resolved by pull request 49171
#49171