[mlir] Remove redundant declarations (NFC) (#166896)

In C++17, static constexpr members are implicitly inline, so they no
longer require an out-of-line definition.

The comments for these variables are also present in:

  mlir/include/mlir/Dialect/Bufferization/IR/BufferizationBase.td

Identified with readability-redundant-declaration.

Author: kazutakahirata

mlir/lib/Dialect/Bufferization/IR/BufferizationDialect.cpp

@@ -21,25 +21,6 @@ using namespace mlir::bufferization;
 
 #include "mlir/Dialect/Bufferization/IR/BufferizationOpsDialect.cpp.inc"
 
-/// Attribute name used to mark function arguments who's buffers can be written
-/// to during One-Shot Module Bufferize.
-constexpr const ::llvm::StringLiteral BufferizationDialect::kWritableAttrName;
-
-/// Attribute name used to mark the bufferization layout for region arguments
-/// during One-Shot Module Bufferize.
-constexpr const ::llvm::StringLiteral
-    BufferizationDialect::kBufferLayoutAttrName;
-
-/// An attribute that can be attached to ops with an allocation and/or
-/// deallocation side effect. It indicates that the op is under a "manual
-/// deallocation" scheme. In the case of an allocation op, the returned
-/// value is *not* an automatically managed allocation and assigned an
-/// ownership of "false". Furthermore, only deallocation ops that are
-/// guaranteed to deallocate a buffer under "manual deallocation" are
-/// allowed to have this attribute. (Deallocation ops without this
-/// attribute are rejected by the ownership-based buffer deallocation pass.)
-constexpr const ::llvm::StringLiteral BufferizationDialect::kManualDeallocation;
-
 //===----------------------------------------------------------------------===//
 // Bufferization Dialect Interfaces
 //===----------------------------------------------------------------------===//