FIx: Remove superfluous arg to __bp_set_ret_value

[glensc]

__bp_set_ret_value() takes just one argument:

• bash-preexec/bash-preexec.sh

  Lines 180 to 182 in a44754f

  	__bp_set_ret_value() {
  	return ${1:+"$1"}
  	}

[glensc]

Seems it's trickery to set $_:

• d056f7d

perhaps it then needs better code documentation, i.e for __bp_set_ret_value() function.

[rcaloras]

@glensc, thanks for submitting the PR.

Seems it's trickery to set $:
Yes, it effectively propagates the $ variable so it can continue to be used. Feel free to close the PR or edit the the comments if you like.