Remove dynamic translation of regression test scripts, step 2.

"git mv" all the input/*.source and output/*.source files into
the corresponding sql/ and expected/ directories.  Then remove
the pg_regress and Makefile infrastructure associated with
dynamic translation.

Discussion: https://postgr.es/m/1655733.1639871614@sss.pgh.pa.us

contrib/dblink/sql/.gitignore

@@ -1 +0,0 @@
-/paths.sql

contrib/dblink/sql/paths.sql

@@ -0,0 +1,23 @@
+-- Initialization that requires path substitution.
+
+-- directory paths and dlsuffix are passed to us in environment variables
+\getenv abs_srcdir PG_ABS_SRCDIR
+\getenv libdir PG_LIBDIR
+\getenv dlsuffix PG_DLSUFFIX
+
+\set regresslib :libdir '/regress' :dlsuffix
+
+CREATE FUNCTION setenv(text, text)
+   RETURNS void
+   AS :'regresslib', 'regress_setenv'
+   LANGUAGE C STRICT;
+
+CREATE FUNCTION wait_pid(int)
+   RETURNS void
+   AS :'regresslib'
+   LANGUAGE C STRICT;
+
+\set path :abs_srcdir '/'
+\set fnbody 'SELECT setenv(''PGSERVICEFILE'', ' :'path' ' || $1)'
+CREATE FUNCTION set_pgservicefile(text) RETURNS void LANGUAGE SQL
+    AS :'fnbody';