sd-varlink: drop pointless bitfield

sd-varlink: drop pointless bitfield

As is often the case, in this case because of alignment, we are actually
not saving any space. With the bitfield we are using one bit of the 8 bytes
allocated, and without the bitfield we are using 8 bits of that.

But we're paying a price in generated code, at every access site to the
field:

$ diff <(objdump -S build/libsystemd.so.old) <(objdump -S build/libsystemd.so.new)
...
       v->protocol_upgrade = false;
-   fa2d2:	48 8b 45 a8          	mov    -0x58(%rbp),%rax
-   fa2d6:	0f b6 90 90 01 00 00 	movzbl 0x190(%rax),%edx
-   fa2dd:	83 e2 fe             	and    $0xfffffffe,%edx
-   fa2e0:	88 90 90 01 00 00    	mov    %dl,0x190(%rax)
+   fa2a9:	48 8b 45 a8          	mov    -0x58(%rbp),%rax
+   fa2ad:	c6 80 90 01 00 00 00 	movb   $0x0,0x190(%rax)