Remove unanchored "error" string from symtab2gb test specs

This string would (also) match the very legitimate output "Building
error trace".

Author: tautschnig

regression/symtab2gb/multiple_symtabs/test.desc

@@ -7,4 +7,3 @@ user.json_symtab library.json_symtab
 ^\[2\] file library.adb line \d+ assertion: FAILURE$
 ^VERIFICATION FAILED$
 --
-error

regression/symtab2gb/single_symtab/test.desc

@@ -7,4 +7,3 @@ entry_point.json_symtab
 ^\[2\] file entry_point.adb line \d+ assertion: SUCCESS$
 ^VERIFICATION FAILED$
 --
-error