Remove useless empty lines from stack traces

This printed things like

```
in function 'f'
        called on standard input

in function 'd'
        called on standard input

in function 'b'
        called on standard input

in function 'a'
        called on standard input

```

As a first step, it removes the empty lines so it's now

```
in function 'f'
        called on standard input
in function 'd'
        called on standard input
in function 'b'
        called on standard input
in function 'a'
        called on standard input
```

See #5434.

src/parser.cpp

@@ -416,8 +416,6 @@ void parser_t::stack_trace_internal(size_t block_idx, wcstring *buff) const {
                 append_format(*buff, _(L"\twith parameter list '%ls'\n"), tmp.c_str());
             }
         }
-
-        append_format(*buff, L"\n");
     }
 
     // Recursively print the next block.

tests/invocation/broken-config-continues.err

@@ -4,4 +4,3 @@ syntax-error
 ^
 from sourcing file $XDG_CONFIG_HOME/fish/config.fish
 	called during startup
-