gdb/testsuite: remove definition of true/false from gdb_compiler_info

gdb/testsuite: remove definition of true/false from gdb_compiler_info

Since pretty much forever the get_compiler_info function has included
these lines:

    # Most compilers will evaluate comparisons and other boolean
    # operations to 0 or 1.
    uplevel \#0 { set true 1 }
    uplevel \#0 { set false 0 }

These define global variables true (to 1) and false (to 0).

It seems odd to me that these globals are defined in
get_compiler_info, I guess maybe the original thinking was that if a
compiler had different true/false values then we would detect it there
and define true/false differently.

I don't think we should be bundling this logic into get_compiler_info,
it seems weird to me that in order to use $true/$false a user needs to
first call get_compiler_info.

It would be better I think if each test script that wants these
variables just defined them itself, if in the future we did need
different true/false values based on compiler version then we'd just
do:

  if { [test_compiler_info "some_pattern"] } {
    # Defined true/false one way...
  } else {
    # Defined true/false another way...
  }

But given the current true/false definitions have been in place since
at least 1999, I suspect this will not be needed any time soon.

Given that the definitions of true/false are so simple, right now my
suggestion is just to define them in each test script that wants
them (there's not that many).  If we ever did need more complex logic
then we can always add a function in gdb.exp that sets up these
globals, but that seems overkill for now.

There should be no change in what is tested after this commit.