[llvm-objcopy] - Remove an excessive zlib::isAvailable() check and dead code.

There are 2 places where llvm-objcopy creates CompressedSection:

For --compress-debug-sections. It might create the compressed section from
regular here:
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/ELF/ELFObjcopy.cpp#L486

All initially compressed sections are created as CompressedSection during reading the sections
from an object:
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/ELF/Object.cpp#L1118
Those have DebugCompressionType::None type and a different constructor.

Case 1 has the following code in its constructor:

if (!zlib::isAvailable()) {
  CompressionType = DebugCompressionType::None;
  return;
}
(https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/ELF/Object.cpp#L267)

We can never reach that code with because would report an error much earlier:
https://github.com/llvm-mirror/llvm/blob/master/tools/llvm-objcopy/CopyConfig.cpp#L480

So the code I am removing is dead. Landing this will address the issue mentioned in https://bugs.llvm.org/show_bug.cgi?id=40886.

Differential revision: https://reviews.llvm.org/D59019

llvm-svn: 355503