Remove out-of-line constexpr static data member definitions

Summary:
```
xplat/folly/crypto/Blake2xb.cpp:26:28: error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Werror,-Wdeprecated]
constexpr size_t Blake2xb::kMinOutputLength;
                           ^
xplat/folly/crypto/Blake2xb.cpp:28:28: error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Werror,-Wdeprecated]
constexpr size_t Blake2xb::kMaxOutputLength;
                           ^
xplat/folly/crypto/Blake2xb.cpp:30:28: error: out-of-line definition of constexpr static data member is redundant in C++17 and is deprecated [-Werror,-Wdeprecated]
constexpr size_t Blake2xb::kUnknownOutputLength;
```

Reviewed By: Orvid

Differential Revision: D62261373

fbshipit-source-id: e71de0d6ac341b717ae92cd17e2a6984f857730b

folly/cli/NestedCommandLineApp.cpp

@@ -47,9 +47,6 @@ ProgramExit::ProgramExit(int status, const std::string& msg)
   CHECK(status_ != 0 || msg.empty());
 }
 
-constexpr StringPiece const NestedCommandLineApp::kHelpCommand;
-constexpr StringPiece const NestedCommandLineApp::kVersionCommand;
-
 NestedCommandLineApp::NestedCommandLineApp(
     std::string programName,
     std::string version,

folly/crypto/Blake2xb.cpp

@@ -22,13 +22,6 @@
 namespace folly {
 namespace crypto {
 
-// static
-constexpr size_t Blake2xb::kMinOutputLength;
-// static
-constexpr size_t Blake2xb::kMaxOutputLength;
-// static
-constexpr size_t Blake2xb::kUnknownOutputLength;
-
 namespace {
 
 // In libsodium 1.0.17, the crypto_generichash_blake2b_state struct was made

folly/test/SocketAddressTestHelper.cpp

@@ -20,15 +20,6 @@
 
 namespace folly {
 
-constexpr const char* SocketAddressTestHelper::kLoopbackAddrIPv4;
-constexpr const char* SocketAddressTestHelper::kLoopbackAddrIPv6;
-constexpr const char* SocketAddressTestHelper::kGooglePublicDnsAName;
-constexpr const char* SocketAddressTestHelper::kGooglePublicDnsBName;
-constexpr const char* SocketAddressTestHelper::kGooglePublicDnsAAddrIPv4;
-constexpr const char* SocketAddressTestHelper::kGooglePublicDnsBAddrIPv4;
-constexpr const char* SocketAddressTestHelper::kGooglePublicDnsAAddrIPv6;
-constexpr const char* SocketAddressTestHelper::kGooglePublicDnsBAddrIPv6;
-
 bool SocketAddressTestHelper::isIPv4Enabled() {
   return isFamilyOfAddrEnabled(kLoopbackAddrIPv4);
 }