Remove unnecessary (and ignored) `const` from return types.
Remove unnecessary (and ignored) `const` from return types.

This fixes a compiler warning that warns us that a `const`
qualifier on return types will be ignored by the compiler.
Note that this only applies to return-by-value; it is of
course still ok to using `const` instead of a reference or
pointer, such as `const std::string&'.

vstakhov/libucl,vstakhov/libucl,allanjude/libucl,vstakhov/libucl,vstakhov/libucl,vstakhov/libucl,allanjude/libucl,allanjude/libucl,allanjude/libucl,allanjude/libucl