GH-49356: [C++] Remove deprecated APIs from v13.0.0 and v18.0.0 (#49171)

### Rationale for This Change

This PR removes C++ APIs that have been deprecated for more than one year, in line with Arrow's deprecation and cleanup policy. All removed APIs were deprecated before January 1, 2025.

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### What Changes Are Included in This PR?

This PR removes **3 deprecated C++ APIs**:

**1. `decimal(precision, scale)` function**
- Deprecated in v18.0.0 (September 30, 2024, PR #43957)
- Replacement: `smallest_decimal(precision, scale)`
- Removed from: `cpp/src/arrow/type_fwd.h`, `cpp/src/arrow/type.cc`

**2. `cuda::DefaultMemoryMapper(device_type, device_id)` function**
- Deprecated in v16.0.0 (April 16, 2024, PR #40699)
- Replacement: `arrow::DefaultDeviceMapper`
- Removed from: `cpp/src/arrow/gpu/cuda_memory.h`, `cpp/src/arrow/gpu/cuda_memory.cc`

**3. `HasValidityBitmap(Type::type id)` global function**
- Deprecated in v17.0.0 (July 11, 2024, PR #41115)
- Replacement: `may_have_validity_bitmap(Type::type id)`
- Removed from: `cpp/src/arrow/type.h`
- Note: Member functions like `ArrayData::HasValidityBitmap()` remain unchanged

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### Are These Changes Tested?

Yes. Verified via full codebase search that all removed symbols have zero remaining usages in the C++ codebase.

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### Are There Any User-Facing Changes?

Yes. Downstream C++ consumers using these deprecated APIs will need to migrate to the replacement APIs listed above.

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* GitHub Issue: #49356

Authored-by: unknown <alimahmoodrana82@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>