Ticket #4087: verilog syntax: removing keyboards attribute, endattribute, strength

Keywords attribute, endattribute, strength are not a reserved keywords in any
verilog standard. I can find a few references to these saying that they
were reserved keywords in OVI LRM version 2.0 (a version of verilog
before it was standardized), but this LRM is not available. The OVI LRM
version 1.0 didn't include these keywords, and none of the IEEE standards
include these keywords. I also tried the syntax highlighter of two
commercial tools, and they don't show attribute/endattribute as keywords.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>

misc/syntax/verilog.syntax

@@ -43,7 +43,6 @@ context default
     keyword  whole  always yellow
     keyword  whole  and yellow
     keyword  whole  assign yellow
-    keyword  whole  attribute yellow
     keyword  whole  begin yellow
     keyword  whole  buf yellow
     keyword  whole  bufif0 yellow
@@ -59,7 +58,6 @@ context default
     keyword  whole  edge yellow
     keyword  whole  else yellow
     keyword  whole  end yellow
-    keyword  whole  endattribute yellow
     keyword  whole  endcase yellow
     keyword  whole  endmodule yellow
     keyword  whole  endfunction yellow
@@ -115,7 +113,6 @@ context default
     keyword  whole  small yellow
     keyword  whole  specify yellow
     keyword  whole  specparam yellow
-    keyword  whole  strength yellow
     keyword  whole  strong0 yellow
     keyword  whole  strong1 yellow
     keyword  whole  supply0 yellow