[LibOS+Pal] manifest: Remove support for loader.exec and sgx.sigfile

Supporting these options complicates the design of Graphene and loading
logic significantly, providing little useful functionality:
- loader.exec:
    - the main user of it were our tests
    - worked only for the first process spawned inside Graphene, as it
      was a unidirectional manifest->binary mapping, so the child
      process didn't know about the corresponding manifest.
- sgx.sigfile:
    - probably all existing usages of it were completely redundant
    - was resolved relatively to CWD instead of the executable location,
      which made it mostly useless

From now on, the correct location of the files is:
- either place the manifest and sigfile next to the binary, with a
  matching name, or
- create a symlink to the binary in the folder where manifests are
  stored and launch it through this symlink

Examples/python-simple/.gitignore

@@ -1 +1,2 @@
 *.pyc
+/python