Remove StringRef::isNullTerminated and StringRef::c_str

Both of them were fundamentally unsafe to use and shouldn't be used
at all.

src/catch2/internal/catch_stringref.hpp

@@ -69,10 +69,6 @@ namespace Catch {
             return m_size;
         }
 
-        // Returns the current start pointer. If the StringRef is not
-        // null-terminated, throws std::domain_exception
-        auto c_str() const -> char const*;
-
     public: // substrings and searches
         // Returns a substring of [start, start + length).
         // If start + length > size(), then the substring is [start, start + size()).
@@ -91,10 +87,6 @@ namespace Catch {
             return m_start;
         }
 
-        constexpr auto isNullTerminated() const noexcept -> bool {
-            return m_start[m_size] == '\0';
-        }
-
     public: // iterators
         constexpr const_iterator begin() const { return m_start; }
         constexpr const_iterator end() const { return m_start + m_size; }