[llvm-exegesis] Remove redundant declarations (NFC) (llvm#166044)

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

This patch removes the unnecessary "const" from the in-class
declaration as well.

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/tools/llvm-exegesis/lib/ParallelSnippetGenerator.cpp

@@ -350,7 +350,5 @@ ParallelSnippetGenerator::generateCodeTemplates(
   return Result;
 }
 
-constexpr const size_t ParallelSnippetGenerator::kMinNumDifferentAddresses;
-
 } // namespace exegesis
 } // namespace llvm

llvm/tools/llvm-exegesis/lib/ParallelSnippetGenerator.h

@@ -28,7 +28,7 @@ class ParallelSnippetGenerator : public SnippetGenerator {
   generateCodeTemplates(InstructionTemplate Variant,
                         const BitVector &ForbiddenRegisters) const override;
 
-  static constexpr const size_t kMinNumDifferentAddresses = 6;
+  static constexpr size_t kMinNumDifferentAddresses = 6;
 
 private:
   // Instantiates memory operands within a snippet.