gdbsupport: remove attempt to define TARGET_WORD_SIZE

gdbsupport: remove attempt to define TARGET_WORD_SIZE

In the gdbsupport configure.ac file, there is an attempt to define
TARGET_WORD_SIZE.  This is done by running grep on the file
../bfd/bfd-in3.h.

The problem with this is, the file bfd-in3.h is generated into the bfd
build directory when bfd is configured, and there is no dependency
between the gdbsupport module and the bfd module, so, for example, if
I do:

  $ ../src/configure
  $ make all-gdbsupport

Then bfd will neither be configured, or built.  In this case
TARGET_WORD_SIZE ends up being defined, but with no value because the
grep on bfd-in3.h fails.

However, it turns out that this doesn't matter; we don't actually use
TARGET_WORD_SIZE anywhere.

My proposal in this commit is to just remove the definition of
TARGET_WORD_SIZE, the alternative would be to add a dependency between
configure-gdbsupport and configure-bfd into Makefile.def, but adding a
dependency for something we don't need seems pretty pointless.