[lldb] Remove redundant condition in watch mask check (NFC) (#94842)

This issue is reported by cppcheck as a pointless test in the watch mask
check. The `else if` condition is opposite to the previous `if`
condition, making the expression always true.

Caught by cppcheck -

lldb/source/Plugins/Process/Linux/NativeRegisterContextLinux_arm.cpp:509:25:
style: Expression is always true because 'else if' condition is opposite
to previous condition at line 505. [multiCondition]

Fix #91223