build: don't shadow debug with "@debug@" in tools build

build: don't shadow debug with "@debug@" in tools build

In 16181cbb (tools: Honor Config.mk debug value, rather than setting our
own), configure doesn't set debug variable anymore. There is, however,
one place that was missed. The file config/Tools.mk.in was still
expecting a @debug@ value from configure. After 16181cbb that value
remained "debug := @debug@" all the time because configure didn't
substitute it.

The consequence was that we couldn't get a debug build even if debug was
set to "y" in Config.mk.

Fix this by removing the stray line "debug := @debug@" in Tools.mk.in.

Reported-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Fabio Fantoni <fabio.fantoni@m2r.biz>
Acked-by: George Dunlap <george.dunlap@citrix.com>