Fix sp-kill-hybrid-sexp bug in REPL-like modes

After killing a full line (i.e. `sp-point-in-blank-line` returns non-nil), `sp-kill-hybrid-sexp` inserts a number of spaces equal to the value of `(current-column)` before the kill was performed. When killing a full line in a mode where the editable line begins at a non-zero column this causes extraneous spaces to be inserted.

Instead, only insert the spaces (if any) necessary to get point back to the original indentation level recorded before the kill.