RFR: JDK-8172714 Remove unused and unexpanded variables from spec.gmk.in

[Magnus Ihse Bursie]

We do not expand the value of @BUILD_HOTSPOT@ and @THEPWDCMD@ anymore in 
the generated spec.gmk.

No-one seems to be suffering from this (and I've double-checked with 
a grep) so we should just remove the declarations.

Bug: https://bugs.openjdk.java.net/browse/JDK-8172714
Patch inline:
diff --git a/common/autoconf/spec.gmk.in b/common/autoconf/spec.gmk.in
--- a/common/autoconf/spec.gmk.in
+++ b/common/autoconf/spec.gmk.in
@@ -274,8 +274,6 @@
  CONFIGURESUPPORT_OUTPUTDIR:=@CONFIGURESUPPORT_OUTPUTDIR@
  BUILDJDK_OUTPUTDIR=$(BUILD_OUTPUT)/buildjdk

-BUILD_HOTSPOT=@BUILD_HOTSPOT@
-
  BUILD_FAILURE_HANDLER := @BUILD_FAILURE_HANDLER@

  ENABLE_GENERATE_CLASSLIST := @ENABLE_GENERATE_CLASSLIST@
@@ -642,7 +640,6 @@
  NICE:=@NICE@
  PATCH:=@PATCH@
  PRINTF:=@PRINTF@
-PWD:=@THEPWDCMD@
  RM:=@RM@
  RMDIR:=@RMDIR@
  SED:=@SED@

/Magnus