[Therion] Remove symbols from legend

[Bruce Mutton]

> Can p_u_foo be hidden from the legend?

Some examples.  Statements need to be in thconfig outside of layouts.

#symbols removed from legends
text en "point u:foo" ""

text en "point station:temporary" ""

text en "line wall:bedrock"      ""
text fr "line wall:bedrock"      ""
text mi "line wall:bedrock"      ""

text en "line border:visible"     ""
text en "line border:temporary"   ""
text en "line border:presumed"    ""
text en "line rock-edge"          ""

text en "area bedrock"            ""
text en "point stalactites"      ""
text en "point stalagmites"      ""

Bruce