To start emacs in a given mode:
(setq initial-major-mode 'hexl-mode)
Delete the first n characters of every line
Mark Ctrl+Space the beginning of first line and move cursor to the last line, and move it to the right n chars.
Then type Ctrl+x r k (kill-rectangle).
How to record a sequence of keystrokes
To start recording, press Ctrl+x ( (kmacro-start-macro).
Type your keystrokes.
To stop recording, press Ctrl+x ) (kmacro-end-macro).
To run the keystrokes you've just recorded, press Ctrl+x e (kmacro-end-and-call-macro)
To exit hexl mode: hexl-mode-exit