doc: Remove section about include guards

doc/developer-notes.md

@@ -1160,16 +1160,6 @@ namespace {
 
   - *Rationale*: Avoids confusion about the namespace context.
 
-- Use include guards to avoid the problem of double inclusion. The header file
-  `foo/bar.h` should use the include guard identifier `BITCOIN_FOO_BAR_H`, e.g.
-
-```c++
-#ifndef BITCOIN_FOO_BAR_H
-#define BITCOIN_FOO_BAR_H
-...
-#endif // BITCOIN_FOO_BAR_H
-```
-
 GUI
 -----
 

test/lint/lint-include-guards.py

@@ -83,7 +83,7 @@ def main():
 
         if count != 3:
             print(f'{header_file} seems to be missing the expected '
-                  'include guard:')
+                  'include guard to prevent the double inclusion problem:')
             print(f'  #ifndef {header_id}')
             print(f'  #define {header_id}')
             print('  ...')