Remove parent's DEJAGNU environment variable

`make report` / `make check` on riscv-gnu-toolchain depends on the
default installation of DejaGnu's `riscv-sim.exp`.  If an user overrides
the DejaGnu environment with `riscv-sim.exp`, many tricks used in
the testing framework in riscv-gnu-toolchain stops working.

So, this commit removes parent's `DEJAGNU` environment variable by
`unexport`ing it.

Makefile.in

@@ -57,6 +57,9 @@ endif
 
 export PATH AWK SED
 
+# "make report" depends on the default installation of riscv-sim.exp.
+unexport DEJAGNU
+
 MULTILIB_FLAGS := @multilib_flags@
 MULTILIB_GEN := @multilib_gen@
 ifeq ($(MULTILIB_GEN),)