remove all deprecated string_span stuff (#1074)

This solves issue #1070 and removes the class `string_span`. The only content remaining in the header file `gsl/string_span` is the `*zstring` types.
This also removes the `string_span_tests.cpp` unit tests as these were only for the deprecated `string_span` class.

Co-authored-by: Werner Henze <werner.henze+gitcommits@posteo.de>

tests/CMakeLists.txt

@@ -200,7 +200,7 @@ add_executable(gsl_tests
     span_ext_tests.cpp
     span_tests.cpp
     strict_notnull_tests.cpp
-    string_span_tests.cpp
+    
     utils_tests.cpp
 )