gdb: Remove unused signal mask

gdb: Remove unused signal mask

In the following commit:

    commit 7feb7d068ae65557ede03c36468ebac61b0939ca
    Date:   Mon May 11 12:08:03 2009 +0000

The last useful uses of normal_mask in linux-nat.c were removed, since
then this variable has sat around being initialised, but never used.
There should be no user visible changes after this commit.

gdb/ChangeLog:

	* linux-nat.c (normal_mask): Delete.
	(_initialize_linux_nat): Don't initialise normal_mask.