runltp: Remove export LTP_DEV="" from setup()

In case that loop device cannot be initialized the LTP_DEV stays set to
empty string which breaks testcases that works with loop device with:

chmod06     0  TINFO  :  Using test device LTP_DEV=''
chmod06     1  TBROK  :  ../include/safe_macros.h:297: tst_device.c:180: stat(,0xbeb70620) failed: errno=ENOENT(2): No such file or directory
chmod06     2  TBROK  :  ../include/safe_macros.h:297: Remaining cases broken
chmod06     3  TBROK  :  chmod06.c:230: chmod(testdir_1) failed: errno=ENOENT(2): No such file or directory

Resolves #23

Signed-off-by: Kamil Rytarowski kamil@semihalf.com
Tested-by: Kamil Rytarowski kamil@semihalf.com
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>