[Demangle] Remove redundant declarations (NFC) (#166895)

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

Identified with readability-redundant-declaration.

Author: kazutakahirata

llvm/lib/Demangle/ItaniumDemangle.cpp

@@ -25,10 +25,6 @@
 using namespace llvm;
 using namespace llvm::itanium_demangle;
 
-constexpr const char *itanium_demangle::FloatData<float>::spec;
-constexpr const char *itanium_demangle::FloatData<double>::spec;
-constexpr const char *itanium_demangle::FloatData<long double>::spec;
-
 // <discriminator> := _ <non-negative number>      # when number < 10
 //                 := __ <non-negative number> _   # when number >= 10
 //  extension      := decimal-digit+               # at the end of string