x86/boot/compressed/64: Remove unused variable

Fix gcc warning:

arch/x86/boot/compressed/pgtable_64.c: In function 'find_trampoline_placement':
arch/x86/boot/compressed/pgtable_64.c:43:16: warning: unused variable 'trampoline_start' [-Wunused-variable]
  unsigned long trampoline_start;
               ^

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lkml.kernel.org/r/1563283040-31101-1-git-send-email-zhenzhong.duan@oracle.com