src/current/compiler/map.xsl: Omit preproc:from from retmap symbols

preproc:sym/preproc:from is used for generating `knownFields` using the
_input_ map, so this has no use for return map values; the map still
produces edges to its dependencies.

The issue is that there are return map entries in some of our systems that
are producing multiple `preproc:from`, but I somewhat-recently modified the
system to support only a single map, to remove dynamic allocation.  This
resolves that problem.

With that said, `knownFields` was created for Liza to know when the
classifier ought to be invoked, to save time.  Back when it was first
introduced ~10y ago, this provided significant savings, however the
structure of our system now is such that nearly every single field invokes
the classifier.

Furthermore, these details should remain encapsulated; if we wanted to make
that determination, we should be provided with a delta, which we could also
use to do incremental classification in the future, if there's an ROI there
after other improvements have been made.

So, eventually, preproc:sym/preproc:from will go away entirely.

DEV-10936

.rev-xmlo

@@ -1,4 +1,6 @@
 # This number is incremented for every compiler change to force rebuilding
 # of xmlo files.
-5
+6
+
+# 6: Return map omit preproc:sym/preproc:from
 

src/current/compiler/map.xsl

@@ -346,6 +346,14 @@
   </call-template>
 </template>
 
+<!-- Omit preproc:from for return map entries, which
+     do not contribute to knownFields -->
+<template match="/*[ @lvmc:type='retmap' ]/lvm:pass" mode="preproc:symtable" priority="6">
+  <call-template name="lvmc:mapsym">
+    <with-param name="name" select="@name" />
+  </call-template>
+</template>
+
 
 <template match="lvm:pass" mode="preproc:depgen" priority="5">
   <preproc:sym-dep name=":map:{@name}" />
@@ -619,11 +627,6 @@
 
   <call-template name="lvmc:mapsym">
     <with-param name="name" select="$to" />
-    <with-param name="from">
-      <for-each select=".//lvm:from">
-        <preproc:from name="{@name}" />
-      </for-each>
-    </with-param>
   </call-template>
 </template>