PO: remove duplicates entries for hh and cc from ALL_PO_SOURCES

PO: remove duplicates entries for hh and cc from ALL_PO_SOURCES

Introduced with version 1.0.1 :
in stepmake/stepmake/C.make
  ALL_C_SOURCES = h, c, y and l files
in stepmake/stepmake/Targets.make
  ALL_PO_SOURCES = $(ALL_C_SOURCES) $(wildcard $(outdir)/*.hh) $(wildcard $(outdir)/*.cc)

hh and cc files having since then been registered through ALL_CC_SOURCES,
there is no need to double their entry.
Even worse, an "out of tree build" results in a doubled location of
parser.yy and lexer.ll (one of them with a full path).