Avoid splitting C string literals with \-newline

Avoid splitting C string literals with \-newline

Using \ is unnecessary and ugly, so remove that.  While at it, stitch
the literals back into a single line: we've long discouraged splitting
error message literals even when they go past the 80 chars line limit,
to improve greppability.

Leave contrib/tablefunc alone.

Discussion: https://postgr.es/m/20191223195156.GA12271@alvherre.pgsql