Remove call to maparg with 4 arguments

Vim versions before 7.3 032 do not support 'maparg(...)' with 4 arguments.
The stock '/usr/bin/vi' on OS X up to and including 10.8.2
returns 1 if the line

    let expr_map = maparg('<cr>', 'i') =~? '\<cr>'

is run, even though this line is wrapped in a 'try' clause

Support for 4 arguments was added in hg commit

    6c05b5e5c1be4ee9b8f96334b7fba7afa832524e