Emacs Command to Delete Up to Non-Whitespace Character

You might try delete-indentation, my favorite command for joining multiple lines into one line. In your example, put the cursor on the line with “second” and hit M-^ twice. Here are the docs:

M-^ runs the command delete-indentation, which is an interactive compiled Lisp function in simple.el.

It is bound to M-^.

(delete-indentation &optional arg)

Join this line to previous and fix up whitespace at join. If there is a fill prefix, delete it from the beginning of this line. With argument, join this line to following line.